The present invention pertains generally to communication systems and more particularly to optical communication systems using fiber optic cables.
With the advent of the proliferation of data processing equipment, communications between user devices such as computers and peripherals is of great interest to the computer industry. Modems (modulation/demodulation units) have been universally used in the industry as a means of communicating between user devices. Although standard modem devices are capable of using standard network and telecommunication systems to transmit data, modems suffer from several disadvantages and limitations. For example, data transmission baud rates may be limited to the particular modem utilized. Although baud rates have increased substantially over the past few years, the speed at which data is communicated via modems is slow when compared to other methods of communicating data. This is due, in part, to the limited bandwidth resulting from the frequency at which modems operate. Moreover, the baud rate at which a modem is designed must be matched to the baud rate of the data processing device to which it is communicating. This additionally limits the versatility of such communication systems. Also, modems are subject to electromagnetic interference and radio-frequency interference (EMI/RFI), and eavesdropping and data link taps. Other disadvantages of modems are set forth in U.S. Pat. No. 4,399,563 issued Aug. 16, 1983 to Greenberg.
Other methods of communicating data have been used to overcome disadvantages and limitations of typical modem devices. For example, microwave links have been used to provide high-speed transmission of digital data between user devices. However, microwave links comprise an expensive means of transmitting data and have physical limitations which rule out the use of such systems in many applications.
The use of fiber optic links overcomes many of the disadvantages and limitations of the prior art by providing a means of communicating data optically which is immune to electromagnetic and radio-frequency interference. However, due to the complexity of transmitting data by fiber optic cables, typical prior art systems are extremely complex. Such systems are expensive and have been unable to economically compete with standard modem devices. Typical fiber optic communication systems are disclosed in U.S. Pat. No. 4,381,881 issued May 3, 1983 to Bell; U.S. Pat. No. 4,399,563 issued Aug. 16, 1983 to Greenberg; U.S. Pat. No. 4,366,565 issued Dec. 28, 1982 to Herskowitz; U.S. Pat. No. 4,289,373 issued Sept. 15, 1981 to Sugimoto et al; U.S. Pat. No. 4,362,358 issued Dec. 7, 1982 to Hafle and U.S. Pat. No. 4,341,438 issued July 27, 1982 to Seki et al, which are specifically incorporated herein by reference for all that they disclose.
A significant disadvantage of typical fiber optic data links is the necessity of using two fiber optic cables so that optical data can be transmitted and received simultaneously. Use of two fiber optic cables essentially doubles the cost of the fiber optic data communication line. To overcome these disadvantages and limitations, various coupling devices have been devised for using a single fiber optic cable. For example, Bell discloses an expensive and complex fiber optic cross-bar switch for automatically patching optical signals. The Bell system requires the use of multiple optical detectors and multiple LED's (light emitting diodes). Greenberg discloses a time division multiplexing scheme in which problems due to reflections in a Y-coupler device are eliminated by disabling each receiver when a corresponding transmitter is transmitting data. In a similar manner, Herskowitz uses angular division multiplexing to allow for simultaneous bi-directional transmission of data over a single optic fiber. Sugimoto et al, Hafle and Seki et al all disclose wavelength multiplexing to enable bi-directional transmission of optical data over a single fiber optic cable. Again, such systems require complex multiplexing schemes which are expensive to implement.